As the drive splines of gearbox gears tend to wear, several different methods are used to minimize this wear. The splines can be case-hardened steel, and either lubricated by a continuous flow of oil or packed with grease. The other system used is a non-lubricated “plastic” spline coupling insert, as an intermediate piece, that is splined on both its inner and outer diameters. This insert fits between the steel splines of the accessory gearbox drive and the spline on the accessory shaft itself.
The plastic spline coupling insert, while not requiring any lubrication, may increase the size of not only the gearbox shaft, but also the bearing that is supporting the end of the shaft. Additionally, the housing also grows in size to accommodate the bearing/shaft. This increase in shaft size may also result in an increase in the size of the shaft seal, resulting in higher seal rubbing speeds, making the seals prone to leakage.